I had an issue with my MTL doing the same thing. My problem was the effects loop. When it happens plug a jack in and out of the send and return and tell me if it fixes it. It will keep happening if it's the same issue as mine. I think just very slightly bending the jack tip fixed mine and it hasn't happened in a year. It was either that or lead dress. I can't remember exactly as I have worked on quite a few amps since then.